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Gravitation and the Waltz of the Planets
From Ancient to modern astronomy
Geocentric view
Heliocentric view
Galileos observations
Keplers laws of
planetary motion
Columbia Launch: October 18,1993
Columbias crew: STS 93
The Greeks tried to EXPLAIN and UNDERSTAND, not just
PREDICT.
To them, the universe was the basically solar system -the Sun,
Earth, Moon and 5 planets known at that time.
They observed that: - over the course of a night, the stars slid smoothly across the sky. - over the course of a month, the moon moved smoothly in the sky relative to the
stars.
- over a course of a year, the Sun progressed along the ecliptic.
- 5 other bodies known to them were Mercury, Venus, Mars, Jupiter and Saturn. Planetes- wanderer----> do not behave in as regular and predictable manner. They seem to speed up and slow down and appear to loop back and forth relative to stars. Sometimes they stop. Sometimes their brightness change.
Prograde motion: Eastward (forward or direct)
Retrograde motion: Westward (backward)
The Geocentric View (Greek Astronomy)
He gave PROOFS that the Earth was
SPHERICAL:
objects all fell towards its center
perpendicular to ground sphere.
noted shadows cast on moon during eclipse were always round.
Earth - at the CENTER of Universe
- is heaviest thing around therefore it shouldn't move.
The Sun and the Moon revolve with
uniform motion around a circle with
Earth at its center.
This simple model could not account for
the variation in planetary brightness and
their retrograde motion.
. Each planet was taken to move uniformly around a small circle, called an epicycle,
whose center moved uniformly around
Earth on a second and larger circle, known
as the deferent.
Greek philosopher Aristotle - (384-322 B.C.) : The Greek astronomers (& philosophers) Aristotle (384 -322 B.C.)
and Ptolemy (127--151 A.D.) created the GEOCENTRIC MODEL to
explain the motions of the Sun, Moon and planets around the Erath.
To account for the paths of Sun, Moon and the five planets known at that time his model required a series of nearly 80 distinct circles.
While complex, IT WORKED ---
used for 1300 years (~13
centuries)
The Geocentric Model
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Nicholas COPERNICUS (1473--1543), a Polish cleric,
argued in favor of :
1. HELIOCENTRIC COSMOLOGY --- THE EARTH IS NOT THE CENTER OF THE UNIVERSE .
2. The center of the Earth is the center of gravity and for the MOON.
3. The other PLANETS REVOLVE around the SUN.
4. The STARS are MUCH FARTHER from the EARTH than is the SUN.
5. STELLAR (and SOLAR) MOTIONS ARE APPARENT: all are due to motion of the earth.
6. PLANETARY MOTIONS are also substantially due to the Earth's motions.
7. RETROGRADE PLANETARY MOTIONS are also a consequence of Earths motion.
THE COPERNICAN REVOLUTION
Copernicus' model still assumed perfectly circular orbits
and did not dispense with epicycles --- but now the main
orbits went around the Sun and they could be smaller.
While this model fit the data available then, it was only
slightly better and certainly not proven.
SO WHY WAS IT ``BETTER''?
HELIOCENTRIC MODEL was SIMPLER and more
BEAUTIFUL than the Ptolemaic geocentric model.
Still, it gained few adherents:
--- went against common sense --- and written in Latin
Retrograde Motion (The Copernican model)
Two scientists Galileo Galilei and Johannes Kepler :
--- made ineradicable imprints in astronomy
--- popularized the Copernican viewpoint
GALILEO Galilei (1564--1642)
He taught mathematics and astronomy at Padua, Italy.
He performed EXPERIMENTS:
e.g., balls rolling down inclined planes and learned about INERTIA.
He is now widely regarded as father of experimental science.
He built a TELESCOPE in 1609 but not for commercial or military use.
The Birth of Modern Astronomy
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Galileo was the first man who POINTED the
TELESCOPE at THE SKY and DISCOVERED: The Lunar surface was irregular: it had mountains, valleys and
crater etc. Moon wasn't a perfect sphere.
Planets looked like disks, stars still looked like
points.
Milky way comprised of many stars --- many
fainter stars could be seen between those visible
to the naked eye.
The Sun had spots --- the most perfect of all heavenly bodies
had blemishes that moved across the face of Sun- Sun rotates.
Jupiter had four moons --- something other than earth had
things going around it. (One of the strongest supports in the favor
of Copernican model and against Aristotle/Ptolemys model)
Venus went through phases (including a full phase, not possible
in Ptolemaic model) -- reflecting light from the sun.
Galileo discovered that Venus, like the Moon, undergoes a series of phases
as seen from Earth. In the Ptolemaic (geocentric) model, Venus would be
seen in only new or crescent phases. However, as Galileo observed, Venus
is seen in all phases, which agrees with the Copernican model as shown.
Venus appears smallest at gibbous phase and largest at crescent phase.
The Changing Appearance of Venus
Galileo also discovered moons in orbit
around the planet Jupiter. This was
further evidence that the Earth was
not the center of the universe.
Jupiter and its Largest Moons
Observations made by Jesuits in 1620
The Moon in Galileos Sidereus Nuncius
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Galileo's Sunspot Drawings
Credit: http://galileo.rice.edu/
He was a German mathematician and an accomplished amateur
observer, who had already accepted the basic Copernican picture.
In contrast to Galileo, he was a pure theorist. His groundbreaking work about planetary motions was principally a collection of his employer, Tycho Brahe.
MARS MOVED
IN AN ELLIPSE, WITH THE SUN AT ONE
FOCUS. This broke
with the longstanding assumption
that combinations of
circles were the only way to
explain things;
after all, a circle is just a
special ellipse,
with ECCENTRICITY = 0.
By 1609 he had also realized that
Mars' orbit
swept out equal areas in equal
times
TYCHO Brahe (1546--1601), a Danish noble, built his own observatory at Denmark named Uraniborg
and designed HUGE instruments
including:
sextants (for measuring angles) astrolabes (for locating positions on the sky) These allowed for PRECISION MEASUREMENTS,
accuracies of about 1 arc min in planetary and
stellar positions were achieved.
Tycho was the first to quote errors along with his
measurements.
Johannes KEPLER (1571--1630)
1. All planets follow elliptical orbits,
with the Sun located at one focus.
Kepler's laws of Planetary Motion
2. Every planet sweeps out equal areas in equal
times as it orbits the Sun.
[In other words, planets move fastest when closest to the Sun (near
perihelion) and slowest when farthest away (aphelion).]
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KEPLER'S
First Law Second Law
Credit: Bill Drennon (Central Valley Christian High School, Visalia, CA USA)
3. The square of a planets orbital period is
proportional to the cube of its semi-major axis
(distance from the Sun).
P = a
2 3
(in earth years) (in astronomical units)
1 Astronomical unit (AU) = 1.5 x 10 Km (150 million Km) 8
Keplers laws are called empirical laws: not based on any physics or mathematics but just on observations.